fast

Fast is a term with several related meanings rooted in the idea of rapidity and endurance. As an adjective, fast describes something moving, operating, or occurring at high speed, as in a fast car or fast railway service. It can also be used as an adverb to describe the manner of action, for example, running fast. In physics, speed is a scalar quantity that measures how far an object travels per unit of time; fast is a qualitative descriptor rather than a formal measurement. Common units of speed include meters per second, kilometers per hour, and miles per hour, with knots used in maritime and aviation contexts. Technologies and practices that increase speed include automotive engineering, aerodynamics, rail systems, and propulsion.

Fast also appears as a noun and verb in dietary and religious contexts. To fast means to abstain from all or some foods and drinks for a period. Fasting is practiced for religious observance, health, or medical reasons, and norms vary by tradition. Physiological effects of fasting include shifts in metabolism and hormonal changes; extended fasts may induce ketosis or autophagy in some individuals.

Historically, societies have pursued faster transport and communication to reduce travel times and increase throughput, a trend that accelerated with steam power, electricity, and internal combustion engines, followed by high-speed rail, aviation, and the rapid transfer of information in the digital age. The word fast originates from Old English fæst, meaning firm or fixed, and its sense of speed developed over time.